Polarization-resolved imaging of light plays an important role in a number of applications, yielding information about the orientation, material type, and roughness of a surface. Imaging polarimeters examine the polarization state of light reflected or emitted from objects, providing information complementary to the intensity and wavelength data provided by most cameras.
Some efforts have been made to provide camera designs that employ passive imaging polarimeters. Early embodiments of this concept involved time-sequenced serial or amplitude-division parallel polarization filtering in the aperture plane of a conventional imaging system. Advances in nanofabrication have led to increased attention on division of focal plane systems in which an array of micro-polarizing elements is monolithically integrated directly on a Focal Plane Array (FPA) sensor. The mechanical robustness, permanent polarizer-to-sensor alignment, and potential for low-cost fabrication provide significant advantages for this approach, while recent advances in image reconstruction algorithms mitigate the artifacts that arise from each pixel looking at a slightly different part of the scene. Aluminum nanowire micro-polarizer arrays have been demonstrated at visible wavelengths, but they have low polarization selectivity (<100:1 at best for red, 50:1 for blue), suffer from substantial pixel-to-pixel optical cross talk, and require extremely fine lithography (70 nm line widths). These systems have examined only linear polarization due to the difficulty in fabricating micro-polarizing structures sensitive to circular polarization. Linear polarization imaging is adequate for passive imaging applications, but substantially limits active imaging where the scene is illuminated with a set of controlled polarizations.